It is often desirable to use printed materials in low ambient light venues. For example, musicians sometimes need to read sheet music in performances at concerts, theatrical productions, clubs and other venues where house lighting may be low or non-existent. Further, it is sometimes desirable to read books and consult printed materials in locations where ambient lighting is insufficient for comfortable reading, as for example, at night in an automobile. Prior attempts at providing a lighting source for such venues have not been entirely successful.
In some prior devices, incandescent bulbs, powered by batteries or AC power have been used. Incandescent sources, while being relatively inexpensive, also have the drawback of relatively low energy utilization efficiency. This low efficiency results in low light level for amount of power consumed, as well as excessive heat production in the bulb. For battery powered devices, batteries are quickly depleted requiring frequent replacement for alkaline batteries, and frequent recharging for rechargeable type batteries. Moreover, AC power lamps have the drawback of requiring a nearby AC power source. An AC power source is often not readily available in locations where a lamp is desirably used, for example in automobiles. Also, a bulky power cord sometimes including a transformer is required. Further, incandescent bulbs have a relative short service life and require frequent replacement.
Other prior devices have used fluorescent bulbs. These devices have an advantage over incandescent devices in that they typically have better overall energy utilization and run cooler. A drawback, however, is the generally bulky and relatively heavy ballast required for fluorescent bulbs. Also, while having a much longer life than incandescent bulbs, fluorescent bulbs contain mercury and other harmful substances, requiring specialized disposal when the bulb is replaced.
In recent years, LED's have emerged as a viable, low power, relatively high brightness light source for portable lamps. Prior compact LED lamps, however, have generally suffered from a number of drawbacks. In these devices, inexpensive LEDs having a relatively low light output are used in an effort to save cost and provide acceptable battery life. In these devices, the light is usually of poor quality even with fully charged batteries, and has poor overall color and temperature characteristics. The quality of light from these devices degrades quickly as the batteries are discharged.
Just as significant is the poor overall energy consumption efficency of prior simple LED driving circuits. These circuits are usually no more than a power source connected directly to an LED with a current limiting resistance inserted in series. These simpler circuits dissipate electrical energy in the form of heat via the series resistance. The energy loss is proportional to the voltage drop across the series resistance. The voltage drop across the series resistance is essentially the difference in the power source voltage and the voltage required to drive the LED's. For a battery power source, the highest energy loss occurs upon utilizing fresh batteries as this is when the power source voltage is at its highest. For example, given a 6VDC power source, 11 ohm series resistance and 3.25VDC LED voltage, total power consumption of the circuit would be 1.5 Watts of which 0.6875 Watts is dissipated as heat across the series resistance and 0.8125 Watts of useful LED light power. This calculates to 54% efficiency.
As a result of the low efficiency of prior LED lamps, frequent battery replacement or recharging is required in battery powered devices to maintain an acceptable light level. AC powered LED lamp devices have been developed which alleviate the problems with battery usage, but these devices suffer from many of the same deficiencies as AC powered incandescent devices.
What is needed in the industry is a compact, battery powered lamp that alleviates the limitations of prior devices.